It is well documented that running the piston as close to the head as possible is beneficial when normally aspirated. I have also read that on nitrous and forced induction motors it is less important and can even increase power with bigger p2h numbers. I built my motor several years ago and the p2h is only .036". It runs OK on pump gas at 10psi with 9:1 cr and 23* timing. I am wondering at what power level is it better to increase p2h clearance? Also when is it good to use a round dish vs a dish shaped more like the combustion chamber?

Any discussion in this area is welcome. I am looking for discussion and not neccessarily info specific to my combo.

 

I think the 'problem' some people have with a tight deck and power adders is because they won't retard the timing far enough and don't have a feel for how sensitive the timing becomes in some circumstances. How one degree, one way or the other can make it or break it.

I think the reason some folks are of the opinion that a larger deck clearance should be used when using nitrous or boost is because a tight deck speeds the combustion rate and reduces the amount of spark advance required, in some cases, dramatically. Many people are superstitious and can't buy into the concept that less spark advance is better, because as any hot-rodder knows, more is better and too much is not enough.

What needs to be understood is the crank angle BTDC of ignition isn't the important detail, the crank angle of maximum pressure ATDC is the important detail, which unfortunately isn't as easy to determine as the spark (with a timing light). For a given engine design the angle ATDC is fixed, carved in stone, and is generally 15 to 20 ° ATDC; it depends mostly on rod to stroke length. If you are willing to accept NACA research from before WW2, for a given engine design the exact ° ATDC peak pressure should occur doesn't vary except for very high boost it moves further ATDC. Boost Engineer likely has some observations from more recent research that could affect that information, but I believe it to be valid.

Ignition timing, on the other hand, has to change to accommodate the faster or slower combustion speed of a particular operating condition, vacuum advance for slow burning lean light load mixtures, pressure retard for high boost, etc, so that the maximum pressure is always as close as possible to the sweet spot ATDC.

Someday (soon, I hope) we will have a cheap way to data log the cylinder pressure and all this will become less of a mystery.

We adjust the timing to locate the peak pressure at that most efficient crank angle ATDC to turn that pressure into torque. A human analogy is the way you find the easiest angle to turn a winch crank handle to pull a boat onto a trailer; some angle is obviously the best.

Several factors affect combustion speed, the rate at which the combustion progresses through given percentages of the cylinder charge. One factor is turbulence, which is affected by deck clearance and the percentage of cylinder area devoted to squish. Pressure is another, and is affected by power adders. The two different phenomena are additive, they both, independently, speed or slow the combustion.

A tight deck and boost can combine to speed the combustion enough to require very little spark advance. Some fuels burn faster than others and require less spark advance. (Perhaps Boost Engineer will divulge how little spark advance hydrogen requires.) There just seems to be a limit to how close to TDC some folks will retard the timing, after all, retarded is, well, retarded'¦..the very word has negative connotations.

Countless hours of research and development time are spent to design combustion chambers that reduce burn time and so require less spark advance. More advance is just more pressure BTDC and wastes energy trying to turn the engine backwards until the piston can start down the hole, less advance is better, least advance is best.

If you have the attention span for it, this is an informative read. http://www.innovatemotorsports.com/forums/showthread.php?t=5125 If you have the Innovate LogWorks software (free on their support page) you can see the logs of the O2 in each cylinder on the engine that initiated the discussion and better understand the conversation. It's about a 632 ci Ford that makes more power at 28° than 32° but the owner insists on using more timing even though it makes less power'¦'¦go figure.


With an engine like yours I would retard the timing (adjust the timing correctly for the circumstances) and use internal coolants (water/alcohol) before I would open up the deck just so I could advance the timing. 'Retard the timing' is another way to say 'put more of the combustion on the power stroke.'

That's nuts, I can't believe I wrote all that.

 

Hey Tuner.. What do you know about the various aftermarkets heads out there.. And their sensativity to timing advance..

More specificly, I've been told that my Brodix Track 1's require more ignition timing for best power.. Than a set of Twisted Wedges.. Running at the same compression ratio..

What's your take on this ??

 

Thanks for writing all that. The discussion I have seen is on speedtalk.com and the motors were for pro heads up stuff. I am not sure how many people read or post over there so I thought I would try and get some discusion here. There was talk of .125" or more with steel rods for the big boys. I know it is light years away from mine, but I would like to step it up before too long. I am shooting for 800-900 range.

 

Jeff, no matter what combustion chamber design, the proper procedure is to find the least spark advance that makes the most power. Every engine has its own magic number, you have to find it. I have experienced the TW heads being happy with less timing than the traditional 'D' shaped chambers but it may have been the particular engine and how it was being used, circumstances alter cases.

I consider between .050' and about .180' a no-fly zone to be avoided like a rash. I don't know but from my limited experience and what I read in Taylor's MIT book and old NACA papers. In the 60's, in factory race engine publications, Ford, GM and Smokey Yunick advised .035' to .040', minimum preferred. Taylor says .005' maximum per inch of bore diameter in operation, which means .035' -.040' at room temp and zero RPM so expansion and stretch are accounted for. I've seen BBC's touch with no harm at 8200 with .032', just leave a witness mark. Taylor, Obert, and NACA all say roughly .060' to .180' is a clearance space that promotes pro-knock conditions by causing formation of pro-knock radicals.

Did you read the Innovate link? It gets into this.

Now that times are modern and all, things are more like they are now than they ever have been, but the physical chemistry is as old as E=MC squared (eternal). I don't think the facts that were gathered 70 years ago on a Pentagon budget for the war effort have changed.

Boost Engineer has been deep in this every day for the last few decades, help me out here.

 

That was an interesting read. I am not sure what you were trying to show here unless it was to not be afraid to lower the timing. I learned that lesson long ago. I usually dial timing in at the track off mph. Same with jetting. I have been surprised more than once on what a motor wants.

 

It would have been easier to have simply said, "I think your .036" deck clearance is about perfect. Use the least advance that makes the most power and internal coolants if necessary." I thought you wanted to know why?

 

LOL. I like your posts and read them all the way through. I am building a motor and am going to go with a zero deck height piston and set it upright about .035-.040".

 

Not to sound sloppy but in a turbocharged platform it doesn't matter much. NMRA DR racer John Kolivas won the championship(2006) with a engine that had the pistons .080 down in the hole and the thing would barely turn with a breaker bar, but would run the number time and time again. I built an engine with a radial dish(1400whp) and I think that it was worth some power. But than you have to wonder if Subaru and Mitsubishi uses a donut dish by mistake.... If you build them closer than .035 PTH your going to see just how good your parts are. I have a Pure Street engine that it "witnesses" the head on #3 and #7, I guess that throw on the crank is a little bigger the the others? But thats an engine that you have to make the most CR you can. Hell... You guess ever pull the head off a SRT neon? The piston must be .250 down in the hole with a dome piston from factory.

It will be nice in a few years when we have more real world data to pull from in regards to spool vs CR, piston design,chamber design and so on.

 

IMO it doesn't make much of a diffrence in a race motor but gives the pump gas motor more detonation resistance and faster burn rate. Makes things a little more efficient...

 

Rereading my last post I can see why it was taken the way it was. It was not intended to offend. I read the link all the way through and have a better understanding of some of the things going on in the chamber. It was a good read and well worth the time it took to read it. It was at times hard to grasp and some things I read several times to get an understanding.

One of my favorite articles I have ever read was in Roadracing World in the early 90's. It was about the early devolpement at Honda when the turbo's went away. The Chemical engineers from Shell worked with the engine department to come up with a fuel and engine package that ran on around 70 octane fuel and low compression ratio. It was not what they set out for, but where they ended up. The point is that sometimes you have to stray away from conventional thinking to advance.

I have also heard of people using shorter than usual rods to get the piston away from head faster in boosted applications. The idea is again to create more room for the very fast combustion in high doses of N2O and high boost engines.

I am not really looking to make changes to my engine, just get some good discussion on what some people are trying.

 

I have been exploring this "Squish" theory a lot lately. From what I have seen/read power can be made with tight, or loose. Most people will tell you to have a zero deck and run a .039" gasket. That as the piston comes up on it compression stroke, the charge will be forced to the center/ combustion chamber and promote a better/cleaner burn. During this the tight (.039 squish) will not allow a burn/flame due to the to tight of a area. To counter this, I have seen well known engine builders use a huge quench area. They say it will get a more complete burn, burn all the way across the piston I really think with todays very efficient combustion chambers, EFI, and cam timming a lot can be done on pump gas. So, what is the answer? Don't get too tight of a squish or you will have pistons kissing the head. Anything mh ore than that, just depends on how much compression you want. With todays strokers, you can run a flat top, the piston in the hole, and a thick gasket and still get 11-1 compression.

 

Just a few comments based on talking to people in the Boost World over the years.

1) The F1 guys claim at really high rpm the flame front cannot get across a 3.55" bore (90mm). Course their idea of really high rpm is quite a bit higher than the normal Blow-thru guys rpm. Think about flame travel

2) A guy named Don Hubbard (friend of mine) who was written up in the old Hugh Macinnis Turbocharger book told me that there is a sweet spot on PTH distance.
There is a "Really Ugly" larger PTH range where detonation is a MAJOR Concern.
There is an even larger PTH where it doesn't seem to matter as far as Detonation is concerned. This is where the guys really drop the piston down in the hole with boosting added.

3) The old NACA reports mention engines that made lots of HP with boost in the 5-6-7 compression ratio range. Hubbard used to claim that 7.5 CR was great with a lot of Boost on the old OFFY Engines. Those old engines weren't much guys, 90 cubic inches, over 1000 hp for 500 miles. So your 500 cid chebby would have to make 5000 hp to match the HP/CID. F-1 guys used to be over 10 to 1 (HP/CID)
during the Turbo wars.

4) Some of the really good offshore (Honda, etc) engines have 4 valves so the old quench head PTH discussion kind of goes out the window.

Tom Vaught

 

tuner was talking about watching cylinder pressure in each cylinder. I have wondered about this alot. They make the Piezioelectric sensors (forgive my spelling) that fit into spark plugs. Such as the PSI plug and the Cal plug I beleive, as well these sensors can read the cylinder pressure and then in conjunction with a program to match the pressure with crank degrees. I called the company that makes the PSI plug and I beleive they start around 1200 a pop and you have to send them the plugs. Obviously this is not in reach with the average sportsman racer, Im just surprised that we dont see alot more data from this area considering that its there if some of the higher end shop are tuning with them. I would love to see a system that included 2-4 plugs or sensors and instructions to drill out plugs of choice and the program to watch it with the degrees for under 2k but it seems far fetched.

 

i'd love to see that too!!

 

I know this is way after the original post was made but I want people to remember that any engine with a true hemi chamber layout (AKA Justin Curry 3000 hp/ John Force 8000hp) have no quench at all. I think in a boosted application it just doesn't matter that much. as a rule of thumb I like to see .035-.045 If it is going to be wider than that make .080 or wider. JMHO

 

On low boost street/strip stuff, tight is better, .060" or less.

On big boost/racier stuff, moving the pistons down makes MORE power and reduces detonation/pre ignition.

There is no "magic" number, like there is no magic cam. It is all in the combo, chamber shape, quench shape, mixture motion, homogeneity etc.

 

We use sensors similar to this one in our Research Engine Testing:

Engine Development Sensor Has High Sensitivity

Amherst , NY (May 4, 2008) - A new pressure sensor for engine research advances the state-of-the-art in simplified, exacting measurement performance from Kistler Instrument. The piezoelectric sensor type 6045A is designed around a unique PiezoStar® crystal element which has a sensitivity of -45 pC/bar. This higher level signal reduces data compromising effects and eases signal conditioning. The crystal element output remains linear at elevated temperatures by resisting thermal influences thus eliminating the need for water cooling. With low thermal shock, in high impedance, charge mode operation this unit meets the demands of test track tuning. Both thermodynamic investigations and knock measurements can be made with a M8 X 0.75 access hole.

The sensor mounting incorporates a shoulder seal design for efficient heat control. The 6045A sensor is compatible with common laboratory charge amplifiers. It is interchangeable with Kistler water-cooled style 6041A. From simplified mechanical support to highly precise data characteristics, this sensor series can establish basic engine performance as well as help optimize engine efficiency parameters.

Kistler is a worldwide supplier of precision instrumenta­tion for the measurement of pressure, force and acceleration serving the R&D, Industrial and OEM Communities.

Kistler Instrument Corporation


Rarely do we use the spark plug type units. PS This is not an ad for this company. Just a source name for more internet searches on the subject.

Tom Vaught